An electrophoretic display device that displays an image by using an electrophoretic phenomenon is generally known. Such an electrophoretic display device includes a display substrate that is transparent, a back substrate disposed in confrontation with the display substrate, and a gap spacer interposed between the display substrate and the back substrate and provided along the periphery of the substrates. A hermetically-sealed space having a predetermined gap is formed between the display substrate and the back substrate via the gap spacer. The hermetically-sealed space is filled with dispersion medium made of colored liquid or gas in which charged particles that are colored spherical particles are dispersed, thereby forming a display section.
With such a configuration, for example, in an electrophoretic display device in which white charged particles are dispersed in dispersion medium made of black liquid, the two substrates generate an electric field in the display section to migrate the charged particles in the dispersion medium to the display substrate side for displaying white color of the charged particles on a front surface of the display substrate, or to migrate the charged particles to the back substrate side for displaying black color of the dispersion medium on the front surface of the display substrate. A desired image is obtained in this way.
With such a conventional electrophoretic display device, the charged particles may agglutinate or aggregate in a part of the display section as time elapses. Further, the charged particles may settle out in the display liquid when an image is not displayed for a long time. If the charged particles agglutinate, aggregate, or settle out in this way, the charged particles cannot migrate sufficiently even when an electric field is generated in the display liquid, leading to problems of degraded image quality such as color heterogeneity in an image and a low contrast of an image.
In order to prevent such degradation of image quality, it is conceivable to apply a higher voltage with electrodes to generate a stronger electric field for preventing agglutinate of the charged particles and the like. With this method, however, there are problems that a power consumption of the electrophoretic display device increases substantially and that handling of the device becomes more difficult because of high voltage.
Hence, Japanese Patent Application Publication No. 2000-321605 discloses a method in which pulse voltages with opposite polarity are applied alternately prior to image formation. With this method, separation of charged particles adhering to a wall surface can be facilitated, thereby promoting electrophoresis of the charged particles.